A four-wheel drive vehicle, which drives the front wheels and the rear wheels together, has been generally known, and a four-wheel drive vehicle that comprises a central differential mechanism has been also known. In the later case, the central differential mechanism is positioned between the front wheels and the rear wheels such that the rotational driving force of the engine is divided by and transmitted through the central differential mechanism to the front wheels and to the rear wheels. In such a four-wheel drive vehicle, axle differential mechanisms are provided, respectively, for the front wheels and for the rear wheels, each axle differential mechanism dealing with the rotational difference occurring between the right wheel and the left wheel. As a result, it is necessary for such a four-wheel drive vehicle to be equipped with a central differential mechanism and front and rear axle differential mechanisms, totaling to three differential mechanisms.
If such three differential mechanisms are provided each separately, then the construction of the power transmission apparatus requires a large number of parts, presenting problems of the transmission apparatus requiring a large size as well as a high manufacturing cost. To solve such problems, there has been a proposal that either one of the front and rear axle-differential mechanisms be constructed together with the central differential mechanism as one integrated unit (refer to, for example, Japanese Laid-Open Patent Publication No. 2003-48441). According to the apparatus that is proposed for constructing the front axle-differential mechanism and the central differential mechanism as an integrated unit, it is expected that the power transmission apparatus be made compact as a whole.
However, in the integrated differential mechanism, which integrates the front axle differential mechanism and the central differential mechanism, each differential mechanism is open type. For example, if one of the four wheels slips and rotates without any load, then most of the rotational driving force is directed to this slipping wheel, so there is a problem that the vehicle as a whole cannot receive sufficient driving force. Furthermore, if the slip of the wheel rotating without any load is controlled by the brake, then it can present another problem that the brake must perform increased work.